The XPathFactory class is not thread-safe. In other words,
it is the application's responsibility to ensure that at most
one thread is using a XPathFactory object at any
given moment. Implementations are encouraged to mark methods
as synchronized to protect themselves from broken clients.

XPathFactory is not re-entrant. While one of the
newInstance methods is being invoked, applications
may not attempt to recursively invoke a newInstance method,
even from the same thread.

newInstance

To find a XPathFactory object,
this method looks the following places in the following order where "the class loader" refers to the context class loader:

If the system property DEFAULT_PROPERTY_NAME + ":uri" is present,
where uri is the parameter to this method, then its value is read as a class name.
The method will try to create a new instance of this class by using the class loader,
and returns it if it is successfully created.

${java.home}/lib/jaxp.properties is read and the value associated with the key being the system property above is looked for.
If present, the value is processed just like above.

The class loader is asked for service provider provider-configuration files matching javax.xml.xpath.XPathFactory
in the resource directory META-INF/services.
See the JAR File Specification for file format and parsing rules.
Each potential service provider is required to implement the method:

The first service provider found in class loader order that supports the specified object model is returned.

Platform default XPathFactory is located in a platform specific way.
There must be a platform default XPathFactory for the W3C DOM, i.e. DEFAULT_OBJECT_MODEL_URI.

If everything fails, an XPathFactoryConfigurationException will be thrown.

Tip for Trouble-shooting:

See Properties.load(java.io.InputStream) for exactly how a property file is parsed.
In particular, colons ':' need to be escaped in a property file, so make sure the URIs are properly escaped in it.
For example:

http\://java.sun.com/jaxp/xpath/dom=org.acme.DomXPathFactory

Parameters:

uri - Identifies the underlying object model.
The specification only defines the URI DEFAULT_OBJECT_MODEL_URI,
http://java.sun.com/jaxp/xpath/dom for the W3C DOM,
the org.w3c.dom package, and implementations are free to introduce other URIs for other object models.

newInstance

Obtain a new instance of a XPathFactory from a factory class name. XPathFactory
is returned if specified factory class supports the specified object model.
This function is useful when there are multiple providers in the classpath.
It gives more control to the application as it can specify which provider
should be loaded.

Tip for Trouble-shooting

Setting the jaxp.debug system property will cause
this method to print a lot of debug messages
to System.err about what it is doing and where it is looking at.

If you have problems try:

java -Djaxp.debug=1 YourProgram ....

Parameters:

uri - Identifies the underlying object model. The specification only defines the URI
DEFAULT_OBJECT_MODEL_URI,http://java.sun.com/jaxp/xpath/dom
for the W3C DOM, the org.w3c.dom package, and implementations are free to introduce
other URIs for other object models.

classLoader - ClassLoader used to load the factory class. If null
current Thread's context classLoader is used to load the factory class.

Returns:

New instance of a XPathFactory

Throws:

XPathFactoryConfigurationException - if factoryClassName is null, or
the factory class cannot be loaded, instantiated
or the factory class does not support the object model specified
in the uri parameter.

setFeature

Set a feature for this XPathFactory and
XPaths created by this factory.

Feature names are fully qualified URIs.
Implementations may define their own features.
An XPathFactoryConfigurationException is thrown if this
XPathFactory or the XPaths
it creates cannot support the feature.
It is possible for an XPathFactory to expose a feature value
but be unable to change its state.

getFeature

Feature names are fully qualified URIs.
Implementations may define their own features.
An XPathFactoryConfigurationException is thrown if this
XPathFactory or the XPaths
it creates cannot support the feature.
It is possible for an XPathFactory to expose a feature value
but be unable to change its state.